1. Field of the Invention
The present invention relates to a compression molding method and mold clamping apparatus that may be suitable for use in injection compression molding, injection press molding, and pressurized press molding of synthetic resin materials and other molding compounds.
2. Description of the Related Art
A compression molding is known as one type of molding methods for molding compounds such as plasticized synthetic resin materials. The known compression molding involves the steps of: parting by a given amount a stationary mold half and a movable mold half that cooperate to provide a mold having a mold cavity defined therebetween, while introducing the molding compound into the mold cavity; and clamping the mold in this state to effect compression molding. By way of specific example, known methods for compression molding of synthetic resin materials include injection compression molding, injection press molding, and pressurized press molding, which may be executed in injection molding of synthetic resin material. The injection compression molding may be performed such that plasticized synthetic resin material is injected to fill the mold cavity defined between the stationary and movable mold halves clamped together, and the mold halves are re-clamped together after permitting a given amount of mold parting due to an injection pressure exerted on the mold upon injecting the material into the cavity. On the other hand, the injection press molding is performed such that plasticized synthetic resin material is injected into the mold cavity, with the stationary and movable mold halves held parted by a predetermined amount, and the mold halves are then clamped. Further, the pressurized press molding may be performed such that a primary molding of synthetic resin material or the like is pressurized.
Compared to general injection molding processes, which merely involve injecting plasticized synthetic resin material into the mold cavity defined by the stationary and movable mold halves held in a clamped state, such compression molding processes are not only more effective in preventing sink marks and other molding defects in moldings, but also afford a number of additional advantages such as improved transfer and strength characteristics, and an ability to mold thin, large moldings with consistently high precision. Thus, in recent years, such compression molding processes have been studied in-an effort to implement in molding methods for products of various kinds.
When carrying out compression molding, factors, such as symmetricity of the mold cavity or bias of the gate location prone to causes the molding compounds filled within the cavity to create reaction force to a mold clamping force or a non-uniform pressure distribution within the cavity, when the stationary and movable mold haves are clamped together. As a result, the stationary and movable mold halves tilt relative to each other so that the two mold halves cannot be clamped in parallel, possibly leading to reduced dimensional accuracy and molding defects of moldings.
Accordingly, a number of proposals have been made to date to inhibit relative tilt of the stationary mold half and movable mold half during the clamping operation in compression molding. JP-A-5-269750, for example, teaches a mold clamping apparatus control method wherein a plurality of mold clamping hydraulic cylinder mechanisms are installed between the stationary mold half and movable mold half, and during the clamping operation, feedback control is executed in such a way that a stroke position of each mold clamping hydraulic cylinder mechanism reaches a target stroke position, while at the same time controlling the stroke positions of the cylinder mechanisms so as to be mutually equal. JP-A-11-179770 teaches another mold clamping apparatus control method wherein a plurality of mold clamping hydraulic cylinder mechanisms are installed between the stationary mold half and movable mold half, and during the clamping operation, feedback control is executed in such a way that detected pressure of one of the mold clamping hydraulic cylinder mechanisms reaches a target clamping pressure, while at the same time controlling stroke positions of the other mold clamping hydraulic cylinder mechanisms to be equal to that of the one of the mold clamping hydraulic cylinder mechanisms, on the basis of detected pressure of the one mold clamping hydraulic cylinder mechanism.
However, the control method according to the former (JP-A-5-269750) has some drawbacks, since mold clamping is controlled simply on the basis of the stroke position in each mold clamping hydraulic cylinder mechanism. Namely, when clamping pressure is particularly high and the distance traveled by the movable mold half, i.e., the change in stroke position of the mold clamping hydraulic cylinder mechanism is small, the effect of the stroke position on clamping pressure is considerable. This makes it difficult to precisely control the mold clamping hydraulic cylinder mechanism at around completion of clamping or during a pressure release operation, for example, resulting in the problem of difficulty in achieving consistent clamping pressure and a tendency for clamping pressure to fluctuate during the mold clamping operation.
With the control device according to the latter (JP-A-11-179770), there is an unavoidable delay between operation of the one mold clamping hydraulic cylinder mechanism and of the other mold clamping hydraulic cylinder mechanisms which follow it, resulting in a tendency for the stationary and movable mold halves to tilt relative to each other. Additionally, clamping pressure is controlled solely on the basis of detected pressure of the one mold clamping hydraulic cylinder mechanism, making it difficult to ascertain clamping pressure acting on the molding overall. If the pressure distribution within the mold cavity be deviated, clamping pressure may differ significantly from preset molding conditions.